Hey Beth, here's the criteria for which channels appear:
To appear in the list, the channel must have at least 1 top-level post within the selected timeframe, and sent by the filtered employee/non-employee group (if applicable)
To have TTFR data displayed, the channel must have at least 1 reply to the post within the selected timeframe, and sent by the filtered group (if applicable). Otherwise, it displays -.
I'll put it in our TODOs to definitely make this more transparent to the end user.